I don't think that anyone can really say that 10 server connections is the most anyone would need. Maybe some people really like to chat a lot, and they need 11 servers. Or 12. Who's to say how many is too many? I think a hard limit is not what we need. Maybe /scid (and /scon) should be limited only to those server windows that were already open when the command was first executed. That would stop an infinite loop of this kind.
If a hard limit is implemented, though, I would think that something like 50-100 would be better. I can't think of any valid reason for having that many server connections at a time, but that doesn't mean that there isn't one. mIRC already seems to give users the benefit of the doubt by not having a set limit, and I like it that way.